Metal halide lamp

ABSTRACT

A high pressure metal halide discharge lamp containing mercury and metal halides including an alkali metal halide and wherein the segregation of alkali metal (sodium) is minimized. In a lamp for vertical operation, the normally asymmetric alkali metal distribution in the vapor phase is offset or reduced by an asymmetric lamp construction using at the lower end one or more of a smaller electrode which runs hotter, a heat-reflecting coating, or a tapered end chamber. This construction drives the condensed metal halides away from the lower end while raising the temperature of the excess halide, and exerts a net force on the alkali metal atoms in the vapor phase to drive them toward the upper end of the lamp. The result is that segregation of the alkali metal is reduced, leading to higher efficiency, improved color and reduced color separation.

United States Patent 1191 Hellman 1 Aug. 14, 1973 [73] Assignee: General Electric Company,

Schenectady, NY.

[22] Filed: Jan. 31, 1972 [21] Appl. No.: 222,253

Related US. Application Data [63] Continuation-impart of Ser. No. 5,759, Jan. 26, 1970,

[75] inventor:

2/1966 Reiling 313/47 Primary Examiner-Roy Lake Assistant Examiner-Darwin R. I-lostetter AttorneyErnest W. Legree et a1.

[5 7] ABSTRACT A high pressure metal halide discharge lamp containing mercury and metal halides including an alkali metal halide and wherein the segregation of alkali metal (sodium) is minimized. In a lamp for vertical operation, the normally asymmetric alkali metal distribution in the vapor phase is offset or reduced by an asymmetric lamp construction using at the lower end one or more of a smaller electrode which runs hotter, a heat-reflecting coating, or a tapered end chamber. This construction drives the condensed metal halides away from the lower end while raising the temperature of the excess halide, and exerts a net force on the alkali metal atoms in the vapor phase to drive them toward the upper end of the lamp. The result is that segregation of the alkali metal is reduced, leading to higher efficiency, improved color and reduced color separation.

10 Claims, 9 Drawing Figures PMENTEB MIR 14 $78 SHEET 1 0F 3 FigZ.

b= TEFMJNAL a: IN] Tl/JL b: TERM/NHL lnvezn tor': Wagne l2. HeLLTnan His Ai lrorneg METAL HALIDE LAMP This application is a continuation-impart of my copending application Ser. No. 5,759, filed Jan. 26, I970, similarly titled and assigned and now abandoned.

BACKGROUND OF THE INVENTION The invention relates to metallic vapor arc lamps using an arc discharge in mercury and metal halide vapors to produce light, and is more particularly concerned with control of the temperature pattern of the arc tube in order to obtain higher efficiency and better color.

The mercury arc lamp has a long life and reasonably good efficiency but relatively poor color rendition due to the yellow-green quality of its' light. A radical improvement in both color rendition and efficiency may be achieved by adding to the mercury one or more vaporizable metal halides under proper control of loading, temperature and pressure, the preferred metal halide additive being sodium iodide, optionally with thallium iodide and indium iodide. Such lamps are described and claimed in US Pat. No. 3,234,421 issued to Gilbert I-I. Reiling, on Feb. 8, I966, entitled Metallic Halide Discharge Lamps and assigned to the same assignee as the present invention. These improved lamps have been referred to as mercury metal halide lamps and more recently they have been termed simply metal halide lamps.

In its general construction and appearance, the metal halide lamp may resemble the conventional high pressure mercury vapor lamp comprising a high melting temperature glass are tube mounted within a glass outer jacket having a screw base at one end. Thermionic main electrodes are provided at the ends of the arc tube which contains a quantity of mercury and metal halides along with an inert gas for starting purposes. Thus, by way of example, one may add sodium iodide along with a lesser quantity of thallous iodide and indium iodide to the mercury to achieve a luminous efficiency in the range of 70 to 80 lumens per watt as against the 50 to 60 lumens per watt range of the ordinary mercury lamp. The lamp has the further advantage of an improved color containing a substantial percentage of red light and which is more pleasing to the eye.

The Reiling patent taught that the coldest portion of the arc tube of a metal halide lamp must always be hot enough during operation to insure that an effective amount of the metal halide is vaporized. This result is achieved by juxtaposing the arc tube wall sufficiently close to the are so that the heat of the arc keeps the wall temperature at the required value. In practice one chooses an arc tube of the proper size having in mind the input wattage and the heat losses. For a given size or wattage, a metal halide lamp generally requires a shorter arc tube than a mercury lamp, and it is generally necessary to raise its temperature. In particular, the ends of the arc tube tend to run cool and their temperature needs to be raised. Various means have been proposed for increasing the arc tube temperature, for instance vacuum in the interenvelope space instead of non-reactive gas, a transparent glass sleeve surrounding the arc tube, radiation reflective coatings on the ends of the arc tube, insulating caps over the ends of the arc tube, and shaping of the arc tube ends into small wells in which the electrodes are recessed.

DESCRIPTION OF DRAWINGS In the drawings:

FIG. 1 shows in side view a metal halide lamp embodying the invention.

FIG. 2 shows are tubes of prior lamps with the initial and terminal temperature distributions.

FIG. 3 shows initial and terminal temperature distributions in arc tubes embodying the invention and utilizing different sizes of electrodes at opposite ends.

FIG. 4 similarly shows the temperature distributions in arc tubes utilizing simultaneously different sizes of electrodes and asymmetric heat reflective coatings.

FIG. 5 shows the temperature distributions in smaller size arc tubes similar to those of FIG. 4.

FIGS. 6a and 6b show the temperature distributions in arc tubes embodying the invention and utilizing different sizes of electrodes and asymmetric end contours.

FIGS. 7a and 7b show plots of the ratio of sodium infrared radiation to mercury green radiation along the length of the arc tube under different end conditions, and under different loading conditions, respectively.

SUMMARY OF THE INVENTION By mapping arc tube surface temperatures, I have found that the conventional techniques for raising the arc tube temperature reach their limit of effectiveness much sooner than necessary. The limit is reached when the hottest part of the arc tube begins to exceed the maximum temperature which the arc tube material can safely withstand. However at such time all other portions of the arc tube are well below the limiting temperature. The are tubes are commonly made of vitreous silica or quartz-like glasses for which l,070 is the upper limit because it is the strain point at which repeated heating and cooling result in fracture. In practice, the operating temperature must not be allowed to exceed l,000 C if a reasonably long life is to be achieved. The lamp contains excess liquid metal halide,

that is more halide than can be completely vaporized under the operating conditions, and the excess tends to collect in the coldest region of the arc tube. The vapor pressure will correspond of course to the lower temperature of the cold spot.

In a vertically operated lamp, the lower end of the arc tube tends to run cold and the upper end tends to run hot. This is in large part due to a convective flow of hot mercury vapor which rises in and around the hot central are from the bottom to the top of the arc tube. Such a flow carries a substantial amount of heat to the top end of the arc tube, raising its temperature. After losing heat to the top end of the arc tube, this mercury vapor flows down along the relatively cool walls of the arc tube down to the bottom end, thus completing the convective flow circuit and lowering the temperature of the bottom end of the arc tube.

In an ordinary mercury lamp this convective flow has no significant influence on the concentration of mercury in the hot central arc. Likewise in the metal halide lamp, the concentration of mercury in the central arc is not affected by the convective flow of mercury vapor. However in a metal halide lamp containing an alkali metal such as sodium, the alkali metal concentration is strongly affected by this flow since the relatively small light alkali metal atoms which are about 1,000 times less numerous in the arc tube than mercury atoms, can easily diffuse out of the hot central arc. These alkali metal atoms will combine with halogen atoms in the cooler portions of the vapor phase near the walls of the arc tube and the resulting alkali metal halide will be swept to the bottom of the arc tube by the returning convective mercury flow. At the bottom of the arc tube the alkali metal halide will be returned to the excess liquid metal halide as a condensate. Thus the combination of convective mercury vapor flow and alkali metal diffusion leads to a continual loss of alkali metal from the hot central are. As a result, the upper portion of the central arc can be severely depleted in alkali metal at distances not far above the excess liquid metal halide condensate. This segregation of thealkali metal reduces the amount of alkali metal radiation (orange and red) from the lamp, and lowers the lamp efficiency since the arc temperature must be higher to excite radiation from the other metallic species remaining in the hot central arc. The heavier and larger metal species such as thallium and indium, and to a lesser extent metal species such as scandium, are not as susceptible to this type of depletion since they cannot diffuse out of the hot central core as rapidly as alkali metals.

The segregation of an alkali metal can be seen in FIG. 7a, where the curve labelled A is a normalized (maximum value adjusted to unity) plot of the ratio of sodium infrared radiation to mercury green radiation along the length of a conventional 400 watt metal halide lamp containing sodium iodide. The infrared radiation of sodium was chosen-to show the effect of segregation since it is not absorbed internally inside the arc tube. Thus the plot is an approximate relative measure of the ratio of sodium to mercury concentration in the hot central are along the length of the arc tube. In this instance it is apparent that the major portion of the sodium is located near the lower end of the arc tube.

Since the alkali metal radiation will increase with increasing arc tube temperature more rapidly than radiation from other metallic species in the lamp due to the greater temperature dependence of the vapor pressure of alkali metal halide overthe excess liquid metal halide condensate, it is possible to increase the total amount of alkali metal radiation simply by operating the entire arc tube at a higher temperature. The result of such operation is shown in FIG. 7b where the curve labelled C is an absolute (not normalized) p lot of the ratio of sodium infrared radiation to mercury green radiation along the length of a conventional 400 watt metal halide lamp containing sodium iodide and operated at 400 watts input power. This plot is then proportional to the ratio of sodium to mercury concentration in the hot central are along the length of the arc tube. It is evident that most of the sodium in the vapor phases located near the lower end of the arc tube. The curve in FIG. 7b labelled D is a similar plot for the same lamp operated at 600 watts input power. The higher are tube temperature has resulted in a higher average sodium concentration while leaving the distribution of sodium essentially unchanged. Thus although the operation at higher power will provide a greater amount of sodium radiation, vapor segregation and color separation remain undiminished. Also the higher are tube temperatures will result in shortening the lifetime of the lamp since are tube temperatures in excess of 1,000 C will be reached sooner.

In order to increase the average alkali metal concentration in the vapor phase inside the arc tube while maintaining satisfactory arc tube temperatures, it is accordingly necessary to reduce the amount of segregation of such species. A means of achieving this objective is to move the location of the excess liquid metal halide condensate from the bottom end of the arc tube to some higher point. If this is done, the region of the vapor below the halide condensate (or the cold spot) will be essentially saturated with alkali metal halide by the downward convective flow of mercury vapor, thus reducing in this region the tendency of alkali metal atoms to diffuse out of the hot central arc to form alkali metal halide. .This change in the cold spot location can be brought about by means of an asymmetric construction such that more heat is generated at the lower end of the arc tube, or less heat is lost from the lower end of the arc tube. Further, the construction may be such that more heat is lost from the top end of the arc tube so that it becomes possible to have a cold spot located at the top end of the arc tube. Thus the asymmetric construction is intended to equalize or even reverse the normally asymmetric temperature distribution of the arc tube while not allowing the hottest portion of the arc tube to exceed a temperature of 1,000 C. One

means consists of using a smaller electrode which runs hotter at the bottom end and a larger electrode which runs cooler at the top. Other means which may be used separately or cumulatively are a heat-reflective coating or a heat-insulating jacket about the lower end of the arc tube. Alternatively an end on the arc tube contoured for a higher temperature such as a tapered end or an end chamber reduced in size may be used. Details of suitable constructions are given in the descriptions of preferred embodiments to follow. By such means, the condensate is driven away from the lower end and up the arc tube walls. This is found to reduce the segregation of the various metal species in the arc tube as evidenced by curve B in FIG. 7a. Additionally, the temperature of the condensate is raised, leading to an even higher average concentration of alkali metal atoms in the vapor phase within the arc tube. The end result is higher efficiency and improved spectral output or color, and reduced color separation.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring to FIG. 1 of the drawings, a mercury metal halide vapor arc lamp 1 embodying the invention comprises an outer vitreous envelope or jacket 2 of ellipsoidal form and having a neck portion 3.- The neck is closed by a re-entrant stem 4 having a press 5 through which extend relatively stiff inlead wires 6,7. These inleads are connected at their outer ends to the contacts of the usual screw type base 8, namely the threaded shell 9 and the insulated center contact 10 and at their inner ends to an inner envelope or are tube 12b.

The inner arc tube 12b is made of quartz or fused silica and has sealed therein at opposite ends a pair of main arcing electrodes, 13 at the base end and 14a at the dome end, plus an auxiliary starting electrode 15 at the base end adjacent main electrode 13. The electrodes are supported on inleads which include intermediate thin molybdenum foil sections 16 hermetically sealed through the flattened ends 17, 18 of the arc tube, commonly referred to as full diameter pinch seals. The main electrodes 13, 14a each comprise a double layer tungsten wire helix wrapped around a tungsten core wire and are activated by thorium oxide which coats the turns and fills the interstices within the helix.

The are tube is supported within the outer jacket by a divided or two-part mount. The upper mount section at the base end comprises a pair of longitudinally extending support rods 21 coming together at their upper ends to form an inverted U which is welded to inlead 6. Metal straps 22 fastened to the lower ends of the rods clamp about pinch seal 17, and right angle braces 23 engage blind notches in the end of the pinch seal to stiffen the assembly. The lower mount section at the dome end comprises longitudinally extending support rods 24 coming together at their lower ends to form a U" to which is attached a springy collar 25 engaging a re-entrant nipple 26 in the dome end of the outer envelope. The lower mount section engages pinch seal 18 through metal straps 27 and right angle braces 28. Main electrode 13 is connected to inlead 6 through strap 29 and rod 21; main electrode 14a at the dome end of the arc tube is connected to inlead 7 through curving wire 31. Starting electrode is connected to inlead 7 through current limiting resistor 32 having a value, for instance of 40,000 ohms. The inter-envelope space is evacuated in lamps of 400 watts size or smaller; in larger sizes such as 1,000 watts, it is filled with inactive gas.

A thermal switch 33 consisting of a bimetal, that is a strip of dissimilar metals bent to a U-shape, is welded at one end to the inlead of main electrode 13. As the lamp warms up, the U-shaped piece opens out and its free end engages the inlead of starting electrode 15. The auxiliary electrode is thereby connected to the adjacent main electrode after starting and during operation of the lamp in accordance with the teachings of U.S. Pat. No. 3,226,597 Green.

The are tube contains an inert rare gas such as argon at a low pressure, for instance at 25 torr, in order to facilitate starting and warm-up. In addition the arc tube contains a fill or dose in the form of liquid droplets in which solid constituents consisting of metal halides may be contained during quiescent non-operating conditions. The arc tube is of such size that the coldest portion of its interior wall is maintained at a temperature no less than about 600 C during operation. The quantity of mercury in the dose is such that upon the attainment of a stable operating condition, the mercury is substantially totally vaporized and exerts a partial pressure within the envelope in the range of one to 15 atmospheres, and it is adjusted to achieve the desired voltage drop at the rated operating current. It is necessary that no mercury remain in the pure liquid state since the operating temperature that is required to volatilize the metal halides is substantially higher than permitted by the foregoing mercury vapor pressure range when liquid mercury is present.

In a preferred embodiment, the other solid constitutents comprise a quantity of sodium iodide in excess of that vaporized at the operating temperature plus smaller amounts of thallium iodide and indium iodide or gallium iodide. Sodium contributes strong yelloworange lines which are broadened into the red when the vapor pressure is sufficient, thallium produces an intense spectral line in the yellow-green at 5,350 A. and indium generates intense spectral lines in the blue at 4,102 and 4,51 l A resulting in a balanced color rendition suitable for general illumination. Proportions of constituents which have been found desirable fall within restricted ranges as follows:

mg./cc. Hg 2.0-l0 Nal 0. l-3.0 TII 0. l-0.5 In! or Gal 0.01-0.15

Prior to our invention, a so-called high dose had been favored. For instance in a 400-watt lamp operating with I35 volt arc drop and utilizing an arc tube 12 of 4.5 centimeters arc gap and about 20 cubic centimeters volume shown in FIG. 2, the filling consisted of Hg, mg; Nal, 40 mg; TM, 4 mg; and InI, 0.75 mg. The electrodes l3 and 14 (designated H15) were both the same size and comprised a tungsten shank having a diameter of 30 mils and a double layer helix made of wire 20 mils in diameter and comprising about nine turns in each layer. To insure sufficiently hot ends during operating, heat reflective coatings 35,36 consisting of zirconium xoide (indicated by speckling) were applied to the ends of the arc tube and adjacent portions of the pinch seals as per U.S. Pat. No. 3,374,377 Cook, and the interenvelope space was evacuated. A typical temperature distribution upon vertical operation at the beginning of its life is mapped in FIG. 2. The quartz is coolest (670 C) next to the lower electrode and hottest (780 C) next to the upper electrode, and is everywhere well below l,000 C.

The temperature of the arc tube rises with age. After it has operated an appreciable portion of its rated life, the envelope has darkened and a loss of sodium may have occurred. The darkening tends to make the arc tube run hotter and in addition the loss of sodium causes a voltage rise across the tube for a given current, which entails an increase in the input wattage tending to raise the temperature further. In FIG. 2 the temperature distribution over the same prior arc tube near the end of life has been mapped; the temperature of the quartz near the lower electrode is now 800 C which is not excessive but the temperature in the vicinity of the upper electrode is now l,O20 C. The high temperature at the upper electrode is close to the softening point of quartz and indirectly sets the limit on the lamps performance because the temperature of 670 C at the lower electrode at the beginning of life is dictated by it and is too low for good color and high efficiency.

ASYMMETRIC ELECTRODES In conjunction with our invention, we prefer to use a somewhat shorter arc tube 12a having an arc gap of about 4.0 centimeters and a volume of about 18 cubic centimeters. Also a so-called low dose is used consisting of Hg, mg; Nal, 16 mg; TlI, 0.9 mg; and InI, 0.16 mg. To reduce the temperature spread between the lower and upper ends of the arc tube, an asymmetrical construction wherein a smaller electrode 14a is provided at the lower end and a larger electrode 13 at the upper end may be used as illustrated in FIG. 3a and 3b. Under similar conditions of voltage, current, and input watts, a smaller electrode will run hotter and raise the temperature of the tube in its immediate vicinity. Since in a vertically operating lamp convection effects naturally tend to make the lower end run cooler, the use of a smaller electrode at the lower end tends to equalize the temperatures throughout the tube and reduce the temperature spread from end to end. By way of example, the previously described H 15 size electrode is used for the upper electrode 13 and a smaller size electrode designated H14 is used for the lower electrode 14a wherein the shank diameter is 22 mils, the wire size in the helix is 14 mils on the inner layer and 17 mils on the outer layer and about eight turns are woundin each layer to form the helix. The temperature distributions at the beginning and near the end of life are shown in FIG. 3a and 317, respectively. The minimum temperature occurring in the vicinity of the lower electrode at the beginning of life has been increased to 735 C and this results in a higher metal halide vapor pressure entailing higher efficiency and improved color. The maximum temperature occurs in the vicinity of the upper electrode near the end of life and is 950 C which is well below the upper limit of 1,070 C for fused silica.

ASYMMETRIC END COATINGS electrode mils The temperature spread between the ends of the arc tube may also be reduced by applying heat reflective coatings asymmetrically to the two ends. For instance the zirconium oxide coating may be removed entirely from the upper end and extended higher at the lower end. In the preferred embodiments of the invention illustrated in FIGS. 1, 4 and 5, the features of different sizes of electrodes and asymmetrical heat reflective coatings have been combined. In FIGS. 1 and 4 corresponding to a 400 watt size lamp, the upper electrode 13 in arc tube 12b is of the larger HIS size while the lower electrode 14a is of the smaller H I 4 size. In FIGS. a and b" corresponding to a 175 watt size lamp, the upper electrode 13a in arc tube 12c is of the H14 size while the lower electrode -b is of a yet smaller size wherein the shank diameter is 18 mils and the wire size is reduced proportionately. A heat reflective zirconium oxide coating 36a has been applied to the lower end of and 4b, and 5a and 5b, respectively. The minimum temperature in the vicinity of the lower electrode at the beginning of life has now been raised to 7505 C for the 400 watt lamp and 780 C for the 175 watt lamp. The maximum temperature in the vicinity of the upper electrode near the end of life is 930 C in both cases and safely below the upper limit for quartz.

' The temperatures mapped on F I65. 44 and 5a indicate a substantially uniform temperature over the arc tubes at the beginning of life and this is desirable condition. Near the end of life, FIG. 4b and 5b indicate a spread of merely 35 to 40 between the ends and this represents a very substantial improvement over the prior lamp represented by FIG. 2b. Table 1 below gives the results from the point of view of lumens output, efficiency and percent red in the spectral output as be tween a prior lamp such as shown in FIG. 2, the lamp of FIG. 3 utilizing asymmetrical electrodes and symmetrical reflective coatings, and the preferred lamp of FIG. 4 using asymmetrical electrodes and asymmetrical reflective coatings.

TABLE 1 MV400 (0 Hour Performance, Vertical Operation) FIG. 2 no.3 FIG.4 Design Design Design Electrode size Symmet- Non- Nonrical symmet- Symmetrical rical Reflective Coating Symmet- Symmet- Nonrical rical symmetrical Output: Lumens 30,000 35,000 40,000

Efi'icacy: Lumens Per Watt 88 I00 Red 2.8 5.5 6.5

It will be observed that in the FIG. 4 design which constitutes the preferred embodiment of the invention also illustrated in FIG. I, the efficacy has been raised from 75 lumens per watt to lumens per watt. The percent red has been increased from 2.8 to 6.5, an increase of 230 percent.

The surprising results shown in Table I are explainable by the reduction in sodium segregation and the increase in efficiency taking place. As previously mentioned, the curve labelled B in FIG. 7a is an approximate relative measure of the ratio of sodium to mercury concentration in an improved lamp embodying the invention in the form shown in FIG. 4 while the curve labelled A is the same type of plot for a conventional metal halide lamp similar to that of FIG. 2. It is evident that the sodium segregation has been drastically reduced by the non-symmetrical construction, as opposed to merely operating the entire arc tube at a higher temperature, as has been mentioned in connection with the plots of FIG. 7b.

It will be observed that in FIG. 4 the reflective coating 36a extends about 5 mm above the tip of lower electrode 14a, and the distance between electrode tips is about 40 mm. Thus the heat-reflective coating blocks off about 12.5 percent of the effective arc tube surface. However, when the output of the lamp of FIG. 3 is compared with that of FIG. 4, an increase in output from 35,000 lumens to 40,000 lumens, a 14 percent gain, is measured. This surprising result is likewise due to the reduction in sodium segregation and increase in efficiency which more than make up for the light blocked off by theheat-reflective coating.

ASYMMETRIC ENDS CONTOURS I Another way of equalizing the temperatures of the ends of the arc tube consists in altering the shape or contour of the arc tube ends. FIG. 6 illustrates an arc tube 12d for a 400 watt size lamp in which the pinch shape; any configuration which reduces the heat loss may be substituted, for instance a well or reduced diameter end chamber at the lower end. The feature of asymmetrically shaped ends may be combined with differently sized electrodes 13 and 14a and a heat reflective coating 36a on the lower end only as illustrated to achieve substantial equalization of temperature at the ends of the arc tube in vertical operation. Average temperatures measured on various lamps are mapped on FIG. 6a for the beginning of life and FIG. 6b for the end of life.

The lamp illustrated in FIG. I is intended for base up vertical operation; by reversing the arc tube end for end relative to the outer envelope, a base down design results. Vertical operation ordinarily includes departures from vertical of as much as 15". With the present lamp, benefits in efficiency and color rendition are obtained with departures from vertical as great as 45 but the maximum terminal temperature may exceed desirable limits for long life. Such departures are comprehended when the lamp is said to be intended for vertical operation.

The effect of raising the temperature at the lower end of a vertical arc tube goes beyond simply increasing the temperature of the halide pool. The ordinary effect of increasing the pool temperature is, of course, to increase the vapor pressures of the metal iodides and this results in increased lumens and a warmer color, that is more red. However as the cold spot is moved higher up the arc tube wall, another effect takes place and this is a reduction in alkali metal segregation which occurs in a vertically burning lamp. Experiments have shown that the time constant for alkali metal concentration changes at the top end of a vertically burning lamp can be much different from the thermal time constant of thesame lamp measured by changes in heavy metal (mercury) atom concentration. The asymmetrical designs of the invention contribute to the relocation of the cold spot upwardly from the lower end of the arc tube and thereby reduce the segregation of the various species resulting in improved color and more uniform color from end to end of the arc tube.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. A high pressure electric discharge lamp intended for vertical operation comprising:

a generally cylindrical arc tube formed of a vitreous high temperature resistant material; a pair of arc supporting electrodes sealed into opposite ends of said tube; said tube containing a filling of mercury which is substantially completely vaporized under operating conditions, of metal halide including an alkali metal, and of inert starting gas; said tube being of such size relative to the arc discharge maintained within it and having an asymmetric construction driving the temperature of the lower end above about 700 C and causing the cold spot to be located above the lower electrode during operation without driving the temperature of the upper end above about 1,000 C;

the location of said cold spot causing excess alkali metal halide to condense on the tube wall above said lower electrode whereby to reduce segregation of alkali metal, improve efficiency and color, and reduce color separation.

2. A lamp as in claim 1 wherein said asymmetric construction resides in at least one of the following features: a smaller electrode at the lower end than at the upper end, a more extensive heat reflective coating on the lower end, and a tube end configuration reducing heat loss at the lower end.

3. A lamp as in claim 1 wherein a heat reflective coating is applied to the lower end only and extends beyond the tip of the lower electrode.

4. A lamp as in claim 1 wherein the alkali metal halide is sodium iodide.

5. A lamp as in claim 1 wherein the alkali metal halide is sodium iodide and a heat reflective coating is applied to the lower end only of the arc tube.

6. A lamp as in claim 5 wherein the heat reflective coating extends beyond the upper end of the lower electrode.

7. A lamp as in claim 6 wherein the filling of metal halide includes thallium and indium.

8. A lamp as in claim 1 wherein the alkali metal halide is sodium iodide, a smaller size of electrode is provided at the lower end than at the upper end, and a heat reflective coating is applied to the lower end only of the arc tube.

9. A lamp as in claim 1 wherein the alkali metal halide is sodium iodide, a smaller size of electride is provided at the lower end than at the upper end, a heat reflective coating is applied to the lower end only of the arc tube, and a tube end configuration reducing heat loss is provided at the lower end.

10. A lamp as in claim 9 wherein the filling of metal halide includes thallium and indium. 

2. A lamp as in claim 1 wherein said asymmetric construction resides in at least one of the following features: a smaller electrode at the lower end than at the upper end, a more extensive heat reflective coating on the lower end, and a tube end configuration reducing heat loss at the lower end.
 3. A lamp as in claim 1 wherein a heat reflective coating is applied to the lower end only and extends beyond the tip of the lower electrode.
 4. A lamp as in claim 1 wherein the alkali metal halide is sodium iodide.
 5. A lamp as in claim 1 wherein the alkali metal halide is sodium iodide and a heat reflective coating is applied to the lower end only of the arc tube.
 6. A lamp as in claim 5 wherein the heat reflective coating extends beyond the upper end of the lower electrode.
 7. A lamp as in claim 6 wherein the filling of metal halide includes thallium and indium.
 8. A lamp as in claim 1 wherein the alkali metal halide is sodium iodide, a smaller size of electrode is provided at the lower end than at the upper end, and a heat reflective coating is applied to the lower end only of the arc tube.
 9. A lamp as in claim 1 wherein the alkali metal halide is sodium iodide, a smaller size of electride is provided at the lower end than at the upper end, a heat reflective coating is applied to the lower end only of the arc tube, and a tube end configuration reducing heat loss is provided at the lower end.
 10. A lamp as in claim 9 wherein the filling of metal halide includes thallium and indium. 